Facsimile keying circuit



y 9, 1953 F. T. TURNER 2,639,321

FACSIMILE KEYING CIRCUIT Filed Jan. 18, 1951 llllll IN 1 "EN TOR F. T. TURNER WWI x ATTORNEY Patented May 19, 1953 FACSI-MILE -KEYING CIRCUIT Frank T. Turner, Hampton Bays, N."Y., assignor to The Western ".Union Telegraph Company, 'New York,N."Y., a. corporation of -NewYork 'Applieation.-'.lanuary 18, 1951,.Serial No.-206;644

10 Claims.

The present invention relates to telegraphic communication by facsimile transmission and more particularly to facsimile transmission-using an electric stylus for scanning a-sheet ofxpaper or like material having markedand. unmarked areasxoifering different resistances in the stylus circuit.

-An oscillator keying circuitcomprising an electricstylusfor scannin -paper or like material having marked. and unmarked areas offering different resistances in the stylus circuit is-characterized bysimplicity of structure and-ease'of adjustment .for producing facsimile signals.

.In tuned circuit oscillators, the quality, or Q, of thetuned circuit places an upperlimit on the rate. at which oscillations canbuild up ordecay. This rate bears a relationship to the natural period-of thetuned circuit, the'natural period being a function of thecircuit Q. If,.for instance, the oscillator were operated at a frequency suitable for radio transmission, the buildupand decay rate would behigh compared to the. facsimile keying .rate, so the oscillator could faithfully follow-the-facsimile signal. However, when it is desired'to. generate a carrier-frequency suitable for wire transmission, this frequency being, for instance, in theorderof 10 to 5000 cycles per second, the build-up and decay rate ofthe ordinary oscillator will be too close to the facsimile -keyingrate so that the oscillator can not faithfully follow the facsimile signal. In order to overcome-this difficulty with an ordinary oscillator, recourse must be had .to a bridge circuit or modulator. vSuch .expedients will result inmore complicated structure and will make adjustment of the circuit more difficult.

Accordingly, it is an object of the invention to provide a facsimile keying circuit suitable for use at relatively low frequencies and in which direct keying of a source of carrier signals may be employed.

.More particularly, itis an object oftheinvention to provide a facsimile keying circuit suitable for useat relatively low frequencies and in which the stylus circuit resistance variationsproduce'd as the stylus scans marked and unmarked areas of paper orlike material are usedtokey directly a source of carrier signals.

Further objects of the. invention will appear from the following description.

According to theinventioirthese objectsare achieved by providing a multivibrator circuit coupled to an electric stylus arranged to scana marked= paper and by. providing means. to prevent oscillations for one value-ref iresistance ;provided 2 by .thepaperin the stylus .circuit and tocause oscillations. for another value of resistance -,provided bythe paperin the stylus circuit. It .is tobe understoodthatthe term paper, as .used in the specification and claims, .is to includenot .only paper as ordinarily understood, but any .view being inexaggerate'ddiagrammatic form for clearness.

Referring now to the drawing and more vparticularly to Fig. 1,.therei is shown thebasic elements of a facsimile transmissionsys'tem embodying the inventionan'dcomprising .two electron discharge .tubes lil and [I connected together in..m.ultivibr'ator circuit arrangement and a. stylus 'llarrange'd to scan a .copy sheet [3.

Asshownin Fig. 2', copysheet 13 consists ofa conducting base 1.4,.Which maybe composed of a material suchas carbon impregnatedpaper, and a topcoating l5 of aporous insulating material capable of being easilypenetratedby the point of a conductive pencil. The marks l6 ma'deby a soft pencil.of.-graphite willpenetrate .through thecoating [Beither by removing the material or filling .the pores thereof. "The marks [6 made .hya pencil Willlbe'in good contact with conducting base 1.4 .and aback contact 11 arranged to press. on conducting base [4. Similar copy may be prepared by typewriting or writing on graphite coated transfer paper as disclosed in copending U. S. patent application Serial '.No. 62,184, filed November 26, 1948.

Asstylus l2 scans copysheet l3,'the scanning resistance of the stylus circuit, which -may be considered as the resistance between stylus I2 and back contact", has twoivalues. One of these values is the relatively smalllor negligible resistance encountered .wliile stylus I2 is resting on markslfi. "The other valueis the high re sistance ..encountered'-.while stylus 42 is resting ontop coating 1 5. The invention-is also applicable to transmissionifrom a copy sheet having insulatingcharacters ona conductive background .as .disclosed in-copending U. S.. pat-ent application Serial No.1 5,568,filed. January :3 1,1943. When using-thisilatter type'of copyxsheetiit is advisable to apply the output of the multivibrator to a signal inverter.

Referring again to Fig. 1, a control grid I8 of tube I is coupled to ground through a resistance I9 and a source of negative bias potential 29. Control grid I8 is also coupled to ground through stylus I2 and back contact I'I. Back contact I'I preferably constitutes the grounded metal cylinder on which the copy sheet is mounted for scanning by the stylus.

When stylus I2 is resting on top coating I5, the resistance of the stylus circuit is very high so that the full negative potential of source is applied to grid I9 to cut off tube I0.

When stylus I2 is resting on a mark I6, the resistance of the stylus circuit is very low, so that grid I8 is, for practical purposes, placed at ground potential. As a result, tube I0 will start to conduct.

An anode 22 of tube I0 is coupled to ground through an anode resistor 23 and a source of positive potential 24. Anode 22 is also coupled to a control grid 25 of tube II' through a capacitor 26. Grid 25 is coupled to ground through a resistor 21. Cathode 28 of tube I I is connected to the cathode 2| of tube III. Both cathodes 2| and 28 are coupled to ground through a cathode follower resistor 29. An anode 30 of tube II is coupled to ground through an anode resistor 3I and a source of positive potential 32. The output of the circuit arrangement is derived from anode 30 through a capacitor 33.

In the quiescent condition of the multivibrator circuit, corersponding to the periods when stylus I2 is resting on top coating I5, tube III will be cut off by the bias voltage from source 20 while tube II will be conducting. When stylus I2 rests on a mark l6, grid I8 will be placed effectively at ground potential and thegrid-cathode voltage The flow of current in tube II] will cause the anode to ground potential of tube III to decrease sharply. This decrease in potential will be repeated at grid 25 of tube II because of the coupling of anode 22 to grid '25 through capacitor 25. Since tube I I is connected as a cathode follower, the cathode to ground potential thereof Will also drop, but by an amount less than the drop in grid to ground Voltage. The drop in voltage across cathode resistor 29 decreases the bias of tube I0, thereby tending to increase the current flow in tube I0. Increase in current flow of tube II) will further decrease the anode to ground potential of tube I0 thereby aiding in cutting ofi tube I I. Once cut off, tube II remains cut off until the charge developed across capacitor 26 leaks off suificiently to raise grid 25 above the cut off potential of tube II, thereby allowing tube I I to conduct. At this time a reverse switching operation occurs, the current in tube I I raising the cathode voltage enough to out off tube II], the increase in anode voltage of tube I0 resulting from the decrease in current therethrough aiding in rendering tube I I conductive. As soon as tube II becomes fully conductive, another switching action occurs, rendering tube IO conductive and tube II nonconductive. These switching actions will continue at a frequency determined by the circuit constants, and in particular the discharging time constant of capacitor 29, until stylus I2 rests on top of insulating coating I5. When this occurs the grid to ground potential of tube III will again be determined by source 20 and tube I0 will remain cut off, preventing further switching operations. Since tube I I is not biased to cut off, when source 20 cuts off tube II], the circuit will always return to its quiescent condition with tube I9 out off and tube I I conducting.

The output voltage is derived from anode 30 and will have a generally rectangular waveform determined by the anode voltage variations of tube II. The output voltage could, of course, be derived from other points in the multivibrator circuit, such as, for instance, anode 22 or grid 25. The output voltage may be sent out directly on the line or used to control additional apparatus.

Copy sheet I3 has been described as providing very high and very low values of resistance. However, it is not necessary that these resistance values be widely separated as it is only necessary that with one value of resistance the multivibrator will be cut off and with the other it will oscillate.

While the invention has been described in a particular form thereof and in a particular embodiment, it is not desired that it be limited thereto for obvious modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. Facsimile transmission apparatus, comprising an electric stylus for scanning a transmitting blank having the subject matter-to be transmitted delineated thereon by marking areas having a resistance substantially different from the remaining areas of said blank, a free-running multivibrator comprising first and second electron discharge tubes each having cathode, control grid and anode electrodes, means to interconnect said cathodes, an impedance element intercoupling said cathodes and ground potential and capacitive means intercoupling the anode of said first tube and the control grid of said second tube, means to suppress oscillations in said multivibrator circuit comprising biasing means coupled to the control grid of said first tube to prevent current flow in said first tube, means to produce oscillations in said multivibrator circuit while said electric stylus scans one of said areas comprising means to reduce the bias on the control grid of said first tube, said latter means including a series circuit arrangement intercoupling the control grid of said first tube and ground potential and comprising said electric stylus and said one area, and means coupled to said multivibrator circuit to derive therefrom an output signal while said electric stylus scans said one area.

2. Facsimile transmission apparatus, comprising an electric stylus for scanning a transmitting blank which has relatively conductive and nonconductive areas for delineating thereon the subject matter to be transmitted, a free-running multivibrator comprising first and second electron discharge tubes each having cathode, control grid and anode electrodes, means to interconnect said cathodes, an impedance element intercoupling said cathodes and ground potential and capacitative means intercoupling the anode of said first tube and the control grid of said second tube, means to suppress oscillations in said multivibrator circuit comprising biasing means coupled to the control grid of said first tube to prevent current flow in said first tube,

means to produce oscillations in said: multivibrator circuit while said electric stylus scans said conductive areascomprising means'to reduce the biason the control-grid or said 'firs't tube, said latter 'means includinga series-circuit arrangement intercoupling the control grid of said first tube and ground 'potential'and comprising said electric stylus and said conductive areas, and means coupled to said multivibratorcircuit to derive therefrom an output signal while said electric stylus scans said conductive areas,

3. Facsimile transmission'apparatus', comprising an electric stylus for scanningatransmitting blank which has relatively conductive and nonconductive areas for delineating thereon the subject matter .to be transmitted, la free-running multivibrator comprising first and second electron dischargectubes each having cathode, control grid and anode electrodes, means tointerconnect said cathodes, an impedance element intercoupling said cathodes and ground potential and capacitative means intercoupling the anode of said first tube and the control grid of said second tube, means to suppress oscillations in said multivibrator circuit comprising a source of negative biasing potential coupled to the control grid of said first tube to prevent current fiow in said first tube, means to produce oscillations in said multivibrator circuit while said electric stylus scans said conductive areas comprising means to reduce the bias on the control grid of said first tube, said latter means including a series circuit arrangement intercoupling the control grid of said first tube and ground potential and comprising said electric stylus and said conductive areas, and means coupled to said multivibrator circuit to derive therefrom an output signal while said electric stylus scans said conductive areas.

4. Facsimile transmission apparatus, comprising an electric stylus for scanning a sheet of paper having a conducting base and an insulating upper surface marked with conducting characters which penetrate substantially to said base, a free-running multivibrator comprising first and second electron discharge tubes each having cathode, control grid and anode electrodes, means to interconnect said cathodes, a resistance element intercoupling said cathodes and ground potential and a capacitive element intercoupling the anode of said first tube and the control grid of said second tube, means to suppress oscillations in said multivibrator circuit comprising a source of negative biasing potential intercoupling the control grid of said first tube and ground potential to prevent current fiow in said first tube, means to produce oscillations in said multivibrator circuit while said electric stylus scans said conducting characters comprising means to place the control grid of said first tube substantially at ground potential, said latter means including a series circuit arrangement intercoupling the control grid of said first tube and ground potential and comprising said electric stylus, said conducting characters and said conducting base, and means coupled to the anode of said second tube to derive therefrom an output signal while said electric stylus scans said conducting characters.

5. Facsimile transmission apparatus, comprising an electric stylus for scanning a transmitting blank which has relatively conductive and nonconductive areas for delineating thereon the subject matter to be transmitted, a free-running multivibrator comprising first and second electron discharge systems each having a cathode element, a control grid and an anode, means to interconnect said- 'cathode elements, 3- an impedanc'e element intercoupling said cathode elementsand ground-potential and capacitati-ve" meanszintercoupling the anode of. said:firstdischarge system and the control. grid ofsaid-second'discharge system, mean's to suppress oscillations in 'said multivibrator circuit .comprising biasing -means coupled tothe-control grid of -saidfirst discharge system to prevent 'curren-t ifiow in said i first discharge system, means 1 to 1 produceoscillations :in said multivibrator circuit while :said electric stylus scans said conducting "areas comprising means i to 1 reduce the bias on'the: control grid of said first discharge system, said latter :means including a -series circuit arrangement intercoupling thecontrol grid of said-first discharge system and ground potential -and comprising said electric stylus 'and said conductive I areas, -and means -'-coupled to said multivibrator circuit to derive therefrom -an "output signal *whi-le :said

electric stylus scans said conductive areas.

6. Facsimile transmission apparatus, comprising an electric stylus for scanning a sheet of paper having marked and unmarked areas having respectively different values of electrical resistance, a multivibrator circuit coupled to said electric stylus and having a quiescent condition and an oscillating condition, means to produce said quiescent condition of said multivibrator circuit while said electric stylus scans said areas having one value of electrical resistance, means including said electric stylus to produce said oscillating condition of said multivibrator circuit while said electric stylus scans the areas having said different value of electric resistance, and means to derive an output signal from said multivibrator circuit during said oscillating condition thereof.

7. Facsimile transmission apparatus, comprising an electric stylus for scanning a sheet of paper having marked areas of low electrical resistance and unmarked areas of high electrical resistance, a free-running multivibrator circuit coupled to said electric stylus and having a quiescent condition and an oscillating condition, means to produce said quiescent condition of said multivibrator circuit while said electric stylus scans said unmarked areas, means including said electric stylus to produce said oscillating condition of said multivibrator circuit While said electric stylus scans said marked areas, and means to derive an output signal from said multivibrator circuit during said oscillating condition thereof.

8. Facsimile transmission apparatus, comprising an electric stylus for scanning a transmitting blank having the subject matter to be transmitted delineated thereon by marking areas having a resistance different from the remaining areas of said blank, multivibrator circuit coupled to said electric stylus and having a quiescent condition and an oscillating condition, means for producing said quiescent condition of said multivibrator circuit while said electric stylus scans one of said areas, means including said electric stylus for producing said oscillating condition of said multivibrator circuit while said electric stylus scans the other of said areas, and means to derive an output signal from said multivibrator circuit during said oscillating condition thereof.

9. Facsimile transmission apparatus, comprising an electric stylus for scanning a transmitting blank which has relatively conductive and nonconductive areas for delineating thereon the subject matter to be transmitted, a free-running multivibrator circuit coupled to said electric stylus and having a quiescent condition and an oscillating condition, means for producing said quiescent condition of said multivibrator circuit while said electric stylus scans said non-conductive areas, means including said electric stylus for producing said oscillating condition of said multivibrator circuit while said electric stylus scans said conductive areas, and means to derive an output signal from said multivibrator circuit during said oscillating condition thereof.

10. Facsimile transmission apparatus, comprising an electric stylus for scanning a transmitting blank which has relatively conductive and nonconductive areas for delineating thereon the subject matter to be transmitted, a multivibrator circuit coupled to said electric stylus and having a quiescent condition and an oscillating condition, biasing means coupled to said multivibrator circuit for producing said quiescent condition of said multivibrator circuit while said electric stylus scans one of said areas, means for producing said oscillating condition of said multivibrator circuit while said electric stylus scans the other of said areas, said latter means comprising a series circuit arrangement for substantially short-circuiting said biasing means and including said electric stylus and said other of said areas, and means to derive an output signal from said 'multivibrator circuit during said oscillating condition thereof.

FRANK T. TURNER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,143,875 Hansell Jan. 17, 1939 2,277,261 Smith Mar. 24, 1943 2,511,093 Atwood June 13, 1950 2,539,797 Smith Jan. 30, 1951 

